Water soluble amine resins are potentially useful in cathodic electrocoating, and numerous efforts to develop resins which solubilize in water without excessive acidity, which do not settle out on storage, and which electrodeposit at the cathode and cure to develope good properties have been carried out. It is particularly desirable to employ a resinous system which will cure at relatively low temperature, such as 350.degree. F., but this has frequently required the presence of blocked polyisocyanates which introduce undesirable expense.
It is also desirable to employ resins which possess a bisphenolic backbone, as is found in epoxy resins, since such resins possess good physical and chemical properties.
An interesting effort in this direction is provided in U.S. Pat. No. 4,001,155 in which a bisphenol is reacted with up to 4 molar proportions of formaldehyde and then with about the same molar proportion of monosecondary amine to provide a Mannich base which is then reacted via its phenolic OH groups with from 0.6 to 1.3 equivalents of epoxide functionality per mol of the diphenol which is reacted with formaldehyde. The resulting amine-functional resin is solubilized in water with the aid of an acid, and is useful in cathodic electrocoating.
In contrast with the prior art, in this invention it is desired to more uniformly distribute the solubilizing amine groups and to thereby increase the proportion of epoxy-functional material which can be incorporated. This improves the stability of the aqueous solutions and dispersions which are formed. More particularly, the solubility is improved and the unreacted monosecondary amine adducts with the diepoxide so as to reduce the conductivity of the aqueous system which is formed. Indeed, prior to reaction with the diepoxide, water solubility with the aid of an acid in this invention is poor, but after reaction with the diepoxide, solubility is excellent, and this is surprising. Indeed, the systems of this invention are more complex than those of the prior art, so the avoidance of gelation is itself surprising.